In recent years, production of disk substrates has increased due to an upturn in the demand for its use as a recording device in a hard disk apparatus. In general, a hard disk apparatus includes a shaft that concentrically rotates one or a plurality of disk-like (i.e. doughnut-like shaped) magnetic recording media having a circular aperture in the central portion (that synchronously rotates if the plurality of media are equipped therein); a motor that rotates at a high speed the magnetic recording media connected to the shaft via bearings; a magnetic head that is utilized for recording and/or recalling the data on the both sides of the magnetic recording medium; a supportive arm that is equipped with the magnetic head; and a head stack assembly that synchronously actuates a plurality of supportive arms whereby the magnetic heads can be moved to any position on the magnetic recording media. In addition, such a magnetic head used for magnetic recording and recalling the data is generally a flying-type head where the head moves over the magnetic recording medium at a predetermined height.
Such a magnetic recording medium is produced by way of forming a magnetic layer, a protective layer, a lubricant layer and the like on the surface of the disk substrate having a circular aperture in the central portion. Aluminum or glass substrates are widely used as the disk substrates. Aluminum substrates have advantages in that the processability is high, and that they are cheap. On the other hand, glass substrates have an advantage in that they have excellent strength, and smoothness or flatness of the surface. In particular, miniaturization and densification of disk substrates have been urgently sought in recent years, and glass substrates that have a smaller surface roughness and that enable densification have attracted all the attention.
As a technique that relates to production of such glass substrates for magnetic recording media, a technique of polishing the inner circumference of a glass substrate having a center aperture has been conventionally proposed (for example, see Japanese Unexamined Patent Application, Publication Nos. H11-33886 and H11-221742). These patent documents describe a technique in which a plurality of glass substrates are layered, and then, they are rotated on the axis, and a polishing brush which rotates to the opposite direction of the glass substrates is inserted into the center apertures while the polishing brush is also moved back and forth in the axial direction whereby the inner circumference of each glass substrate can be polished.
The above-described glass substrate for the magnetic recording medium is attached to a motor shaft inside the hard disk apparatus. If its aperture diameter is even slightly smaller than the diameter of the shaft, it is impossible to connect the magnetic recording medium thereto. On the other hand, if its aperture diameter is lager than the diameter of the shaft, then, the magnetic recording medium is connected eccentrically to the shaft, and this causes malfunctional fluctuation thereof when the magnetic recording medium rotates at a high speed. Therefore, a high level of accuracy is required in processing the aperture diameter of glass substrates for magnetic recording media.
Furthermore, as methods of examining the aperture diameter of a disk substrate having a circular aperture in the central portion thereof, (1) a method wherein the diameter of the aperture is analyzed by way of image analysis based on the picture of the aperture taken with a CCD camera; (2) a method wherein an aperture diameter is measured with an inside diameter-inspecting apparatus using two or more stylus probes (for example, see Japanese Unexamined Patent Application, Publication No. H7-198303); and (3) a method wherein the diameter is measured by way of inserting a plug gage into the aperture have been used conventionally.
However, the above-mentioned method (1) has disadvantages in which the measurement takes a long time, and that the aperture diameter is measured erroneously as smaller than the actual size if the substrate inclines even slightly. The above-mentioned method (2) also has disadvantages in which the measurement accuracy thereof is inferior, the measurement takes a long time, and scratches may be present on the substrate where the stylus probes touch. The above-mentioned method (3) is not suitable for the process control since the measurement takes a long time, and it is difficult to quantify the inspected results. Moreover, the method (3) has a disadvantage in which it is likely to cause scratches on the inspected substrate. In particular, the methods (2) and (3) require a long time for the measurement, and therefore, it is difficult for the method to measure all numbers of disk substrates.